Acoustic logging systems are routinely used in the oil and gas industry to measure formation acoustic properties of earth formation penetrated by a well borehole. These properties include the compressional and shear velocities of the formation, which are subsequently used to determine a variety of formation parameters of interest such as porosity and pore pressure. Additionally, acoustic logging systems are used to produce acoustic images of the borehole from which well conditions and other geological features can be investigated. Other applications of acoustic logging measurements include seismic correlation and rock mechanic determination.
The downhole instrument or borehole “tool” of an acoustic logging system typically comprises one or more sources of acoustic pressure or “transmitters”, and one or more acoustic receivers. The transmitters and receivers are typically spaced axially on the body of the tool. Multiple transmitters and/or receivers can also be disposed at different radial positions around the tool. A portion of the energy emitted by the one or more transmitters propagates through formation material surrounding the borehole, and is subsequently detected by the one or more receivers. Receiver response is then used to determine properties and parameters of interest.
Frequencies used in acoustic LWD tools are typically within the 2 to 20 kiloHertz (KHz) range. In order to improve accuracy and precision of measured acoustic information; it is desirable to employ one or more transmitters that have the highest acoustic pressure output at the desired frequency.
Logging-while-drilling (LWD) and measurement-while-drilling (MWD) tools impose severe limitations that affect the energy and frequency output of an acoustic transmitter disposed within the wall of the tool and operating at a desired frequency. Some of these limitations are discussed briefly in the following paragraphs.
If the transmitter comprises piezoelectric crystals, the acoustic pressure output of an acoustic transmitter is proportional to the surface area of the transmitting element. In order to maximize the amount of energy reaching the borehole environs and minimize the propagation of acoustic energy along the tool, it is preferred to dispose the transmitter as near as possible to the outer periphery of the tool. It is, therefore, desirable to dispose the transmitter within a recess in the outer surface of the tool housing wall. An LWD tool housing is typically a drill collar. For structural reasons, it is necessary to restrict the depth, azimuthal and axial dimensions of any recess in the tool wall. These structural recess restrictions therefore govern the maximum dimensions of a transmitter that can disposed within the wall of an LWD tool and, therefore, restrict the acoustic energy output of the transmitter.
The frequency of a piezoelectric crystal element is a function of the geometry of the crystal element. Stated another way, the dimensions of a piezoelectric crystal transmitter determine the frequency output of the transmitter. Considering the discussion in the previous paragraph, a transmitter configured to optimize acoustic energy output within tool structural restrictions may not be configured to obtain the desired frequency output requirements. Conversely, a piezoelectric crystal dimensioned to achieve the desired frequency output may limit the acoustic energy output of the transmitter.
In summary, the transmitter must be configured to operate within a tool in harsh borehole conditions. Structure required to operationally dispose the transmitter within the wall of the tool imposes additional transmitter dimensional restrictions that, in turn, affect energy and frequency output of the transmitter.
In view of the brief background discussion, there is a need for a transmitter with optimized acoustic pressure output, with output frequency optimized to fall within a desired frequency range, and with a physical configuration suitable to meet structural restrictions of LWD and MWD logging systems.